This may be a stupid question but........Im trying to figure out how many gallons are in my friends pool that I'm helping her slam. I thought I could figure it out by how many gallons of chlorine I had added to get her to her slam level but math is not my strong point. So any help is appreciated.

She thinks the size is a 20x40 IG Grecian. Shallow end is 4' deep end is 8'. But she is not positive.

So I added:

4 gallons of chlorine 10%

Raised her FC to 13.

Is it possible to figure out the gallons this way?